If half of SC was underwater, either the land rose, the ocean subsided, or a combination of both. If the ocean levels receded, I would assume that the water was trapped in the form of ice at the polar caps and mountain caps.

As the average global temp rises, trapped water in the form of ice ice melts into the oceans raising the level/height of the oceans.

Photos of Mount Fuji in Japan taken over the past 100 years shows a gradule decline in the ice pack. This is just one example; there are many others.